This invention relates to a method of producing a cam for a jointed camshaft by sintering a compact made from a sintering powder, which compact is calibrated after the sintering process and is then quenched and tempered.
In order to be able to easily produce camshafts from a material corresponding to the respective loads acting thereon, cams are known to be produced by powder-metallurgical methods, and after a heat treatment, for instance by hardening and tempering, by quenching from the sintering heat or by nitriding, nitrocarburizing or plasma-nitriding, to be connected with a steel shaft by usual joining methods. In order to satisfy increased precision requirements, the sintered cams joined with the steel shaft must, however, be subjected to extensive rework by means of grinding, to ensure that not only the outer cam contour, but also its position with respect to the steel shaft lie within the predetermined close tolerances. Despite a substantial consideration of the shrinking behaviour of the cam compact during the sintering process and an extensive calibration of the sintered compact, it was not possible in the case of increased demands as to the dimensional accuracy to maintain the close tolerances to be required in this connection under the conditions of a series production of the cams without regrinding the outer contour of the cam.
It is therefore the object of the invention to improve a method of producing sintered cams for jointed camshafts of the above-described kind such that reworking the cams by means of grinding can be omitted even under the conditions of a series production.
This object is solved by the invention in that the compact is pressed, sintered and calibrated corresponding to a desired contour, which differs from the desired contour of the cam contrary to the distortion produced during quenching and tempering, and that then the calibrated compact is given the desired contour of the cam as a result of the distortion produced during quenching and tempering.
The invention is based on the knowledge that due to an uneven mass distribution over the periphery of the cam the heat treatment during the quenching and tempering of the calibrated cams leads to a distortion which is not uniform over the periphery of the cam and is disadvantageous for its dimensional accuracy, which distortion can be compensated by a corresponding contour of the compact made from the sintering powder. Prior to quenching and tempering, the desired contour of the sintered and calibrated compact must therefore differ from the desired contour of the cam contrary to the distortion caused by the quenching and tempering process, so that the deformation produced during quenching and tempering involves an adaptation of the contour of the compact to the desired contour of the cam. Since the cams are in addition clamped for boring the throughhole for the shaft at the shell, the more precise actual contour of the heat-treated cam leads to improved conditions for properly clamping the cams and thus a more precise alignment of the throughhole with respect to the outer contour of the cam, which altogether leads to the fact that even under the conditions of a series production cams having a high dimensional accuracy can be produced without having to subject the cams connected with the shaft to an extensive rework.
The inventive method of producing a cam will now be explained in detail with reference to the drawing, wherein: